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Old 06-29-2014, 08:41 AM   #1
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Interesting MPG Observations Running House Generator

I've often heard folks say that running the house generator to get good air conditioning instead of the dash would either be a wash or even lead to better MPG. It makes logical sense but I hadn't really tested it out much. So, in the last 6 weeks and 2500 miles I decided to do that especially since I had my generator given the 1000 hour service and know it is in top running shape.

Some background data. I have a Cummins 400 ISL and 7.5KW Onan Quiet Diesel generator. I also have a 2800W PSW inverter that runs a residential refer. All things being normal when docking on the generator running both AC units I typically see 30 amps which is about 1/2 the max which translates into about .5 GPH used by the generator. When driving we only run the front AC and see 15-16 amps which should be close to .25 GPH. We lock the fan on high and set the thermostat @ 70* when on the road.

Here is a list of things that I have seen in the last 6 weeks/2500 miles compared to 6 months/4500 miles prior to the last 6 weeks. Also, keep in mind that the previous 6 months were in much cooler weather than that last 6 weeks. We often ran on the vent air with occasional use of dash AC. I have data for some items while others observations are anecdotal.

1. MPG based on fuel put in the tank that would include fuel used by the generator is slightly up from around 6.4 to 6.8. That doesn't sound like much but when I also factor in the cool weather early this year and not even running the dash AC all the time, that is good all by itself. However, in the last 6 weeks I have at least 24 hours of boon docking use of the generator.

2. The average MPG on my command center which only considers what the computer says I have burned now showed closer to 8.3 - 8.5 ave MPG per tank when prior to that it was showing more like the 7.5ish MPG range on average. It always seemed generous compared to my fuel filling method.

3. Water temp is now pretty solid in the 200-202* range. When I ran the AC it was more in the 185-190* range. I suspect that the AC caused the radiator fan to go to max and cool things down more but use more horse power. According to Big Mike at Spartan the higher temps are GOOD! I also notice I get to the higher temps quicker.

4. Transmission fluid temps now range between 202-206*. With the dash AC on they would stick a bit closer to the water temp and normally be about 2* higher than water and stick pretty close to that. Under normal conditions it still runs close to 202* most of the time but it is not unusual to see 206* for short periods of times. I especially notice that the lag in temp drop when the engine cools down quickly after a long down hill run. In fact, I sometimes see a slight rise in transmission fluid temp at the beginning of a down hill before it starts to trend downward. That is a bit curious but my guess is that the engine cooled quick enough that the fan shut down and reduced air flow over the transmission cooler.

5. Cool down prior to shut down takes a tad bit longer. With the dash AC on I could see water and transmission fluid temps reduce fairly quickly on high idle after pulling into parking after a long run. I would run high idle for about a minute. It now takes about 3 minutes before I see water temps get below 200* where before I would see that temp drop pretty quickly after idle was set. I would also see the transmission fluid temps drop fairly quickly. So, depending on how hot it is outside I see water at about 195-197* within 3 minutes but transmission temps hold 200-204ish at the point I shut down. None of this is distressing at all, just something I have noted. If I was really concerned I could turn on the dash AC a few minutes prior to planned shut down and let that do its thing.

All in all the data seems to indicate that I am getting better MPG including generator fuel burn when driving when I run the house generator to cool us instead of the dash AC. It also seems to be enough of an offset to cover a few hours of generator use when boon docking.

Seems like a win-win to me.
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Old 06-29-2014, 12:14 PM   #2
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While I'm driving a gasser, I was also curious about this. We ran with the dash AC for 650+ miles and got 7.15 mpg. The way back (which was warmer) ran the gen set and roof ACs, got 7.09 mpg on the same mileage. Stopped over night and ran gen for 5 hours with AC, next tank showed 6.98 mpg. Much more comfy with gen running ACs than the dash. No more than I lose, I'll keep it.

My engine temperature runs warmer with dash AC. I can't give exact temps as it just runs on the higher side of normal.
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Old 06-29-2014, 12:29 PM   #3
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My engine temperature runs warmer with dash AC. I can't give exact temps as it just runs on the higher side of normal.
The dash air condenser , sits in front of the radiator , on your gasser , so when operating heats the air going through the grill before it gets to the rad. Your engine will run warmer and the rad fan will cycle more often , the rad fan will be one of the biggest drains on power and mileage. 20+ HP at full howl.
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Old 06-29-2014, 01:14 PM   #4
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So far, it stays cool enough the fan doesn't run unless I'm climbing a good incline, and almost to the top. It's usually off and cooled back down before the decline is over with on the other side. I wish I could see actual temps, cause it fluctuates significantly in the normal range while driving.
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Old 06-29-2014, 01:28 PM   #5
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Scangauge was a eye opener for me. Real temps, oil pressure, trans temp is just a few. Gauges were a feeble guesstamation at best!
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Old 06-29-2014, 02:05 PM   #6
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The one drawback I see is all the extra hours you are putting on your generator. As long as you don't end up with a major repair on it your money ahead, one big problem and you'll never make it up with fuel savings.
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Old 06-29-2014, 02:22 PM   #7
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The one drawback I see is all the extra hours you are putting on your generator. As long as you don't end up with a major repair on it your money ahead, one big problem and you'll never make it up with fuel savings.
Like I said earlier, I just had a major 1000 hour service completed and basically, every major component connected to the motor is replaced. Also, it is pretty clear that generators like to be run. The only thing that I see offsetting any "savings" is the 150 hour service being done more often which is oil, filter and cleaning the spark arrestor.
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Old 06-29-2014, 02:25 PM   #8
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The one drawback I see is all the extra hours you are putting on your generator. As long as you don't end up with a major repair on it your money ahead, one big problem and you'll never make it up with fuel savings.
Actually he is doing his generator a favor by running it. The more you run it the better it will run. This helps keep the generator output section running at best. You would actually end up doing less maintenance on generator that is used a lot than one that is hardly used.
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Old 06-30-2014, 09:35 AM   #9
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I almost always run my generator when driving down the road when it is hot so that it is exercised and that everyone in back stays cool with the a/c. I also run my dash air because I don't like being hot and there is too much windshield heat to allow good cooling in the drivers seat area. I might try installing a shower curtain behind the drivers seat to see if that will help. Has anyone don't that?
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Old 06-30-2014, 01:02 PM   #10
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I have the two fans that sit atop my windshield, which I assume were for defogging, running on a hot day with the dash air. It seems to improve the comfort level. We have yet to run the gen while on the road, it's just the two of us and we are both comfortable with the dash air, so far.

We we boondock, as we will be doing next weekend, odds are we will run the gen set most days but not night time when it's cooler by the sea.

Can't wait....
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Old 06-30-2014, 01:13 PM   #11
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When it's hot out i will run the gen set and the roof ACs. I never think about the hrs that are being put on the unit. Running it is good for it as opposed to keeping it off. Plus I'm not planning on keeping the MH forever so what's he point of having low hrs on it?
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Old 06-30-2014, 02:47 PM   #12
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I have an 8 KW Onan. The manual says it burns:

No load .13 gph; half load .5 gph, and full load 1 gph.
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Old 06-30-2014, 03:04 PM   #13
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so.. I also have an 9KW Onan, .. assuming it is at full load to run 2 AC/s at 1 gph.. and diesel is $4.00 per gal. I would definitely pay $4.00 per hour to have comfort for DW and any other passengers.. and to keep myself cool as an added benefit. Hours of use on the generator? - thats what it's made for. If it is only at 1/2 load .. then I'm smiling!
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Old 06-30-2014, 04:28 PM   #14
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I really wasn't trying to get into the wisdom of running the genny for AC or longevity per se. I was just trying to toss out some figures on the affects I have seen since I started doing that more often.

So far, the numbers are good.
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